1. Field
The presently disclosed subject matter relates to an optical semiconductor device such as alight emitting diode (LED) including III-V group semiconductor compound gallium nitride (GaN).
2. Description of the Related Art
Generally, in an optical semiconductor device including a p-type GaN semiconductor layer, an n-type GaN semiconductor layer and an active semiconductor layer sandwiched by the p-type GaN semiconductor layer and the n-type GaN semiconductor layer, small-incident-angled light emitted from the active semiconductor layer directly or indirectly incident to a light extracting face (upper face) of the semiconductor layers at an incident angle smaller than the critical angle except for its Fresnel component is extracted from the light extracting face. However, large-incident-angled light emitted from the active semiconductor layer directly or indirectly incident to the light extracting face at an incident angle larger than the critical angle is multiply reflected between the light extracting face and its counter face of the semiconductor layers to propagate traversely within the semiconductor layers. Finally, the large-incident-angled light is absorbed by the semiconductor layers, so that the large-incident-angled light cannot be extracted from the light extracting face. Thus, the light extracting efficiency would be decreased.
In order to improve the light extracting efficiency, a first prior art optical semiconductor device is provided with an uneven trapezoidal-sectional structure formed on the counter face of the semiconductor layers (see: JP2006-332383A and JP2007-095744A). The sloped face of the uneven trapezoidal-sectional structure would change the reflection angle of the large-incident-angled light to convert it into small-incident-angled light, thus increasing the ratio of small-incident-angled light to large-incident-angled light. Note that JP2006-332383A relates to a flip-chip (facedown) type optical semiconductor device, while JP2007-095744A relates to a face-up type optical semiconductor device.
Also, in order to improve the light extracting efficiency, a second prior art optical semiconductor device is provided with a silicon oxide layer, a transparent electrode layer and a reflective metal layer formed on the counter face of the semiconductor layers (see: JP2008-98336A). In this case, the silicon oxide layer and the transparent electrode layer totally-reflect the above-mentioned large-incident-angled light toward the inside of the semiconductor layers, and the reflective metal layer reflects the above-mentioned small-incident-angled light transmitted into the silicon oxide layer and the transparent electrode layer toward the inside of the semiconductor layers. The transparent electrode layer also serves to inject currents into the active layer. Note that JP2008-98336A relates to an optical GaAsInP device.
FIG. 10 is a cross-sectional view illustrating a comparative example of the flip-chip type optical semiconductor device which would be obtained by combining the first prior art optical semiconductor device with the second prior art optical semiconductor device.
In FIG. 10, formed on a growing sapphire substrate 1 are an n-type GaN layer 2, an active layer 3 and a p-type GaN layer 4. Also, a silicon oxide layer 5 is formed on the p-type GaN layer 4 to totally reflect large-incident-angled light, and an uneven trapezoidal-sectional p-type GaN layer 6 is formed on a face of the p-type GaN layer 4 where the silicon oxide layer 5 is not formed. The sloped face of the uneven trapezoidal-sectional p-type GaN layer 6 changes the reflection angle of large-incident-angled light to convert it into small-incident-angled light. Further, a transparent electrode layer 7 is formed on the entire face, and a reflective metal layer (p-side electrode layer) 8 is formed on the transparent electrode layer 7 to reflect light transmitted through the silicon oxide layer 5 and the transparent electrode layer 7. Still, an n-side electrode layer 9 is formed on an exposed portion of the n-type GaN layer 2. The transparent electrode layer 7, the reflective metal layer 8 and the n-side electrode layer 9 also serve to inject currents into the active layer 3. Note that the growing sapphire substrate 1 may be removed at a post-stage process, if necessary.
In order to operate the optical semiconductor device of FIG. 10, a drive voltage is applied between the reflective metal layer (p-side electrode layer) 8, i.e., the transparent electrode layer 7 and the n-side electrode layer 9 to inject currents into the active layer 3. In this case, it is assumed that p-type impurities are doped uniformly in the uneven trapezoidal-sectional p-type GaN layer 6. Therefore, the carrier density in the uneven trapezoidal-sectional p-type GaN layer 6 is uniform. As a result, as illustrated in FIG. 11A, currents flow through shortest paths to minimize the electrical resistance thereof. Thus, the currents are concentrated in the roots of the uneven trapezoidal-sectional p-type GaN layer 6. In this case, since the p-type GaN layer 4 is highly electrically-resistant and very thin, the traverse spread of the currents within the p-type GaN layer 4 is so small that the currents flow through only circled parts of the active layer 3 above the roots of the uneven trapezoidal-sectional p-type GaN layer 6. Thus, the current-spreading length L1 of the circled parts of the active layer 3 where electrons and holes are recombined to emit light is so small that the light distribution is non-uniform which degrades the light output characteristics. Also, the drive voltage would be increased, and the reliability would be degraded due to the breakdown of the current-concentrated semiconductor layers 2, 3 and 4.
Note that, as illustrated in FIG. 11B, the spreading length L1 is dependent upon the thickness “g” of the p-type GaN layer 4, not the thickness “h” of the uneven trapezoidal-sectional p-type GaN layer 6. That is,L1∝g. 
In FIG. 11B, note that “i” is an interval between portions of the silicon oxide layer 5.
In FIG. 10, in order to relax the current concentration and increase the current-spreading length L1 of the active layer 3 to substantially increase the light emitting region, after the formation of the uneven trapezoidal-sectional p-type GaN layer 6, the silicon oxide layer 5 may be removed to increase the contact area between the transparent electrode layer 7 and the p-type GaN layer 4. In this case, however, a step for removing the silicon oxide layer 5 is necessary which increases the manufacturing cost. On the other hand, since the silicon oxide layer 5 has a smaller index of refraction than those of the other materials of the optical semiconductor device to effectively reflect small-incident-angled light without absorbing it, it is preferable that the silicon oxide layer 5 be left.